In recent years, display apparatuses have attained remarkable progress. As a result, various display apparatuses such as liquid crystal displays and EL displays are briskly introduced into office automation apparatuses such as PCs and word processors. Each of these display apparatuses has a sandwich structure in which a display device is between transparent conductive films.
Silicon-based semiconductor films constitute the mainstream of switching devices for driving the above-mentioned display apparatuses, due to their excellent stability and processability, as well as their high switching speed. These silicon-based thin films are generally prepared by the chemical vapor deposition (CVD) method.
However, if amorphous, the silicon-based thin films have the disadvantage that, since the switching-speed is relatively low, they cannot display an image when an attempt is made to display images of high-speed animated films or the like. For a crystalline silicon-based thin film, the switching speed is relatively high. However, heating at a temperature of 800° C. or higher or heating by means of a laser is necessary for crystallization, thereby requiring a large amount of energy or a number of steps during the production. Although a silicon-based thin film is excellent also as a voltage element, it suffers from a problem that the characteristics thereof may vary with time when an electric current is passed therethrough.
As a material or the like for obtaining a transparent semiconductor film which is more improved in stability than silicon-based thin films and has a light transmittance equivalent to that of an ITO film, a sputtering target composed of indium oxide, gallium oxide and zinc oxide or a transparent semiconductor thin film composed of zinc oxide and magnesium oxide has been proposed (Patent Document 1, for example). A transparent semiconductor film composed of indium oxide, gallium oxide and zinc oxide, or a transparent semiconductor film composed of zinc oxide and magnesium oxide has the characteristic that it can be etched very quickly with a weak acid. However, these transparent semiconductor thin films are etched with an etchant for a thin metal film. Therefore, when etching a thin metal film on these transparent semiconductor thin films, the transparent semiconductor thin film may be etched simultaneously with the thin metal film. Accordingly, these transparent semiconductor thin films are not suitable when a thin metal film on a transparent semiconductor film is selectively etched.
A film containing crystalline indium oxide, particularly a polycrystalline film, tends to generate oxygen deficiency. Accordingly, having a carrier density of 2×10+17 cm−3 was considered to be difficult even if an oxygen partial pressure during film formation is increased or an oxygen treatment or the like is performed. Therefore, almost no attempt has been made to use this polycrystalline film as a semiconductor film or a TFT.
Patent Document 2 discloses a semiconductor film of indium oxide having a bixbite structure which is obtained by incorporating a positive divalent metal oxide into indium oxide, with an attempt of decreasing the carrier density by incorporating a positive divalent metal oxide. However, in the case of the positive divalent metal oxide, an impurity level may be formed within a band gap of the energy band structure of the bixbite structure, which may decrease the mobility.
Patent Document 1: JP-A-2004-119525
Patent Document 2: WO2007/058248
An object of the invention is to provide a thin film transistor having an indium oxide-based semiconductor film which can allow only a thin metal film on the semiconductor film to be selectively etched.